1. Technical Field
The inventive concepts disclosed herein relate to a fully differential signal system, and more particularly, to a fully differential signal system operating stably and having high performance with a common mode feedback circuit.
2. Description of the Related Art
A fully differential signal system is widely used for an integrated circuit design. When the fully differential signal system is used, an integrated circuit having a high power supply rejection ratio characteristic may be implemented. Moreover, even if noise occurs in a system on chip circuit, the fully differential signal system may transmit a proper signal without being affected from the noise. Furthermore, when the fully differential signal system is used, an even order harmonic distortion that degrades the accuracy of a signal may be reduced.
In general, the fully differential signal system includes a common mode feedback circuit. The common mode feedback circuit is used for constantly maintaining a DC operating point of the fully differential signal system. If the common mode feedback circuit is not included, especially, in a system operating at a low voltage, a DC operating point may leave a saturation region due to a mismatch between elements, a fluctuation in power supply, and a process error. That is, when the common mode feedback circuit is not included, the fully differential signal system may not operate appropriately. When the fully differential signal system includes the common mode feedback circuit, it may become insensitive to a mismatch between elements, a fluctuation in power supply, and a process error.
In order to constantly maintain a DC operating point of the fully differential signal system including the common mode feedback circuit, a DC gain of a system loop needs to be sufficiently large. In addition, in order to prevent the decrease of an operation speed of the fully differential signal system, a unity gain bandwidth of the fully differential signal system needs to be greater than a frequency of an input signal. That is, the unit gain bandwidth of the fully differential signal system needs to be sufficiently large. Additionally, a common mode feedback circuit which is capable of processing all signals included in a swing range of an output signal of the fully differential signal system needs to be designed. The above conditions may be satisfied by adjusting the gain and the bandwidth of an amplifier as well as a proper circuit design.
Furthermore, in order to properly operate the fully differential signal system including the common mode feedback circuit, loop compensation for a stable operation of a system needs to be performed. In addition, the performance of the fully differential signal system needs to be maintained when the common mode feedback circuit is connected. If the fully differential signal system operates unstably or its performance is deteriorated, this means that the fully differential signal system is not designed properly.